1. Field of the Invention
The present invention relates in general to an optical system and method for manufacturing the same. More specifically, the present invention relates to an optical system for spatially controlling light polarization and method for manufacturing the same.
2. Description of Related Art
To keep pace with rapid technical advances in semiconductor integration, there is a need to increase the resolution of an optical system used to manufacture semiconductor devices. Rayleigh's equation in the following Equation 1 introduces a fundamental strategy to enhance resolution (Wmin) of the optical system.Wmin=k1λ/NA  (1)
Thus, to get a high resolution, the wavelength λ of light and process factor k1 should be decreased, and the numerical aperture NA of the system should be increased. As can be seen in FIG. 1, after much effort, the wavelength of light used in an exposure device has been decreased considerably from a G-line of a mercury arc lamp (436 nm) in 1982 to an argon fluoride (ArF) laser wavelength (193 nm) of today. Recent research shows that the wavelength is going to need to be as low as a fluoride dimmer (F2) laser wavelength (157 nm) in the near future. Moreover, since the exposure process has been supported by improved photomasks and lenses, better photoresists and process controls, and more powerful resolution enhancement techniques (RET), the process factor k1 has been decreased from above 0.85 in 1982 to below 0.45 today, with continuing improvement expected in the near future, as shown in FIG. 2.
Meanwhile, the graph in FIG. 1 shows a steady increase in the NA, for example 0.3 for G-line in 1982, 0.6 for a krypton fluoride (KrF) laser (248 nm) in 1998, and 0.7 for an ArF laser (193 nm) in 2002. The increase in the NA is expected to be continuous until an extreme ultra violet (EUV) laser (13.5 nm) is available. As long as an immersion technique in conjunction with current wavelengths of 193 nm or even 157 nm provides sufficient resolution, these wavelengths are expected to continue to be used in semiconductor exposure devices. However, recent inquires have predicted a fundamental loss of transverse magnetic (TM) image contrast caused by a large NA.
The graphs in FIG. 3A and FIG. 3B respectively illustrate a relationship between NAs and image contrast of transverse electric (TE) and TM polarization, as discussed in the article by Timothy A. Brunner, et al., “High-NA lithographic imagery at Brewster's angle,” Proceedings of SPIE, Vol. 4691, Optical Microlithography XV, (July 2002), pp. 11-24. More specifically, FIG. 3A illustrates how image contrast changes with respect to the interference between two coherent beams when the two beams are positioned respectively at the center and the edge of the pupil defining the NA. FIG. 3B illustrates how image contrast changes with respect to the interference between two coherent beams when the two beams are positioned respectively at opposite edges of the pupil defining the NA.
Referring to FIGS. 3A and 3B, the image contrast of a TE polarized light is 1 for every NA, but the image contrast of a TM polarized light decreases as the NA increases. The decrease in the image contrast of TM polarized light is more evident when two beams are arranged at opposite edges of the pupil, as for a powerful RET, e.g., alternating phase-shift masks (PSM). For example, as shown in FIG. 3B, when the NA is 0.71, the image contrast of the TM polarized light decreases to 0, and when the NA is 1, the image contrast of the TM polarized light decreases to −1. When the image contrast of the TM polarized light becomes −1, the TM polarized light and the TE polarized light are offset. Since the TM polarized light does not contribute significantly to the exposure of the photoresist, there is a need to develop an optical system capable of selectively using the TE polarized light in the exposure process using a large NA.
However, as shown in FIG. 4, when a photomask has master patterns 1 with different directions, light polarized in one direction can be absorbed by the photomask. In such case, the intensity of the polarized light reaching the surface of the semiconductor substrate might not be strong enough to expose the photoresist. Therefore, to utilize polarized light for the exposure process, it is necessary to control the polarization state of the light according to its position.
In summary, while an exposure system with a larger NA is required to manufacture highly integrated semiconductor devices, it is also important that the exposure system be able to control the state of polarization of a light beam, especially when the NA exceeds a designated size. However, considering that a variety of different directional patterns are to be formed in the exposure process, an optical system for spatially controlling light polarization is required.